Hi !
When dataset.cache_files is empty (i.e. when your dataset comes from python objects, not from data from your disk), the map() method doesn’t know where to write the resulting dataset.
In this case, you have to pass cache_file_name=<path/to/resulting/cache/file.arrow> manually to map().
For the subsequent calls to .map(), you won’t need to specify this anymore, since it will store the cache files in the same directory as the path your provided in the first place.